1. Field of Application
The following description relates generally to telecommunications systems and wireless communications systems.
2. Prior Art
A typical wireless cellular network comprises many cells, with one or more base stations at each cell. A mobile user within a cell communicates with its serving base station of the cell. Since the locations of mobile users within a cell are random, the quality of the channel between a mobile user and its serving base station can vary significantly. For example, consider the two mobile users 120 and 122 in FIG. 1. Both mobile users 120 and 122 are in the same cell served by base station 112. Mobile user 120 is very close to base station 112, thus the quality of the channel between mobile user 120 and base station 112 can be fairly good. High data throughput can be achieved between mobile user 120 and base station 112.
On the other hand, mobile user 122 is at the edge of its serving cell and much further away from base station 112. In the downlink channel in which base station 112 sends signal to mobile user 122, the strength of the downlink signal that mobile user 122 receives attenuates more due to the increased distance. Further, since mobile user 122 is also closer to neighbor cells, it is also subjected to much stronger interferences of the signals from base stations 114 and 116 of neighbor cells. Consequently, the downlink channel quality can be very poor at the cell edge. In the uplink channel where mobile user 122 sends signal to base station 112, the uplink signal of mobile user 122 is also corrupted by other mobile users 124 and 126 in neighbor cells. Thus at the cell edge, the uplink channel quality can also be very poor. As a result, the data throughput at the cell edge can be much lower than the peak data rate achievable when a mobile user is in the very proximity of a base station. The low data throughput at the cell edge averages down the overall data throughput of the entire cell, thus significantly reducing the network performance.
Recently, in an effort to improve the network performance in terms of the data throughput, multipoint broadcast or multipoint transmission schemes have been introduced to wireless cellular networks. Refer to FIG. 1, where mobile user 122 is connected to base station 112, mobile user 124 to base station 114, and mobile user 126 to base station 116. Without the multipoint transmission, each base station would communicate to its respective mobile user individually. Thus for mobile user 122, the signals it hears from base stations 114 and 116 appear as noises or interferences. If, as shown in FIG. 1, mobile user 122 is at the cell edge, then the interferences from base stations 114 and 116 can be much stronger than the signal from base station 112, and consequently mobile user 122 suffers much poorer communication quality, resulting in much lower data throughput.
A multipoint broadcast scheme aims at increasing the cell-edge performance and can be described as follows. Refer to FIG. 1 where an example multipoint broadcast system can be identified. Base stations 112, 114, and 116 form a set of collaborating multipoint broadcasters. Mobile users 122, 124, and 126 form a set of recipients in the multipoint broadcast system. In the multipoint broadcast system in FIG. 1, base stations 112, 114, and 116 transmit the combinations of the signals intended for mobile users 122, 124, and 126. For each base station, the combination “weight” for each mobile user signal can be different. Through elaborate algorithms, the signals are combined at each base station in such a way that when the transmitted signals from base stations 112, 114, and 116 arrive at mobile user 122, the signals for mobile users 124 and 126 are cancelled out or minimized, while the signal for mobile user 122 is maximized or enhanced, thus the signal quality of mobile user 122 improves significantly. Similarly, mobile users 124 and 126 will also see significant improvement in the quality of their respective signals. The combining of the signals at each base station is commonly referred to as “pre-coding”. The combining weights for each mobile-user signal and for each base station constitute the elements in a so called “pre-coding matrix”.
The signals from base stations in a cellular network are broadcast in nature. Thus a multipoint transmission scheme creates a set of multipoint broadcast channels. With the ability to completely cancel the interference, and to create clean channels for each mobile user, multipoint broadcast channels are shown to have a capacity, a measure of the data throughput of the network, several times that of the traditional cellular networks. Multipoint transmission schemes have been adopted by advanced versions of LTE (long-term evolution, of the currently deployed third generation wireless cellular networks), under the name of “coordinated multipoint transmission”, or CoMP. The name follows from the fact that neighboring base stations coordinate to achieve multipoint broadcast.
While multipoint broadcast can bring tremendous benefits to wireless cellular networks, its performance depends critically on availability of the downlink-channel information at the collaborating base stations. Consider the downlink multipoint broadcast in FIG. 1. For the purpose of interference cancellation, each of the base stations must have the channel information on all downlink channels between a base station and a mobile user. Since there are three base stations and three mobile users involved, there are nine such channels in total in the multipoint broadcast system in FIG. 1. How the base stations get the downlink-channel information depends on the duplex pattern between the uplink and the downlink of the network.
Two duplex patterns exist in cellular networks. One is frequency-division duplex (FDD). In FDD, the uplink and the downlink are assigned to two different frequency bands, and are active simultaneously. This is illustrated in FIG. 2. The other duplex pattern is time-division duplex (TDD). In TDD, the uplink and the downlink share the same frequency band, so in time domain, the uplink and the downlink are active in a non-overlapping or an alternating fashion. This is illustrated in FIG. 3. There also exists the concept of single-channel, full-duplex wireless systems, in which full duplex communications between two wireless devices (a base station and a mobile user, for example) take place in one single frequency band. Its widespread use, however, may still have to wait until certain critical issues, such as self-interference cancellation and limited dynamic range, can be successfully solved in practice.
In an FDD wireless network, a base station is able to estimate the uplink channels from the signals of mobile users which it serves, while a mobile user is able to estimate the downlink channels from the signals of serving base stations. The uplink and downlink channels are generally different since they are in different frequency bands, so the downlink channel is typically considered to be unknown to base stations. To enable multipoint transmission, each mobile user has to feedback the downlink-channel information to the serving base stations via uplink channels. The data rate required for the feedback, however, can be extremely high, which takes up a significant, and, in many cases, a majority portion of the uplink channel capacity. Such a loss in uplink capacity diminishes or even negates the benefits of multipoint transmission.
Moreover, the feedback data needs to be reliably recovered by the base station. Strong error-control coding will have to be applied to the data carrying the channel information. The operations of coding and decoding will introduce coding/decoding delays. The stronger the code in error-correcting capability, the longer the delay. If the channel changes during the coding and decoding, the channel data received by base stations will be outdated.
In a TDD wireless network, the uplink and the downlink share the same frequency, therefore the uplink channel and downlink channel are closely related. Assuming the base stations and the mobile users have phase-synchronized carriers, i.e., the phase difference between the carriers of the base station and the mobile user is zero, then the uplink and the downlink channels are identical (this also applies to the case of non-zero but known phase difference between the carriers, since the uplink and downlink channels can be made to be identical by de-rotating the known phase difference). When the base station estimates the uplink channel, the downlink channel becomes available automatically. Due to the mobile nature of the cellular network, the channels change over time. So strictly speaking, the uplink and the downlink channel are not exactly identical since the uplink and the downlink transmission occur at different times, as shown in FIG. 3. However, if the cycle of uplink and downlink pattern is relatively short compared to the rate of change of the wireless channels, as is typically the case, the difference between the uplink and the downlink channels will be so small so that the uplink and the downlink channels can be considered to be the same.
Since a base station can only estimate the uplink channels from the mobile users which it connects, each base station in a multipoint broadcast system will exchange the estimated channel information with other collaborating base stations. The exchange typically takes place via high-speed backbones that connect all base stations, such as Ethernet or optical fiber. For example, in FIG. 1, base station 112 estimates uplink channels between base station 112 and mobile user 122, between base station 112 and mobile user 124, and between base station 112 and mobile user 126. Base station 112 will send information on the above channels to base stations 114 and 116, and will receive information on other channels between the mobile users and base stations 114 and 116 from base stations 114 and 116, respectively. The exchange is conducted via high-speed backbone connection 110. After the exchange, each base station will have the same global channel information for forming the pre-coding matrix for multipoint transmission.
In practice, however, the phase difference between the carrier of a base station and the carrier of a mobile user always exists and is random. Thus the uplink channel observed by the base station and the downlink channel observed by the mobile user are no longer the same. One way for the base station to acquire the downlink-channel information is the mobile-user feedback via the uplink channel, as is the case in a FDD wireless network. The feedback overhead on the uplink channel, however, diminishes the uplink capacity even more in a TDD network, since with TDD the uplink capacity has already been reduced by the shared downlink. Channel feedback also suffers from the coding and decoding delay as it does in FDD.
There are sophisticated and expensive approaches that force the base station and the mobile user to be locked in phase. For example, all base stations and mobile users can be synchronized with a GPS (Global Positioning System) reference signal. However, this would require high quality, and therefore high cost, RF (radio frequency) components, and elaborate signal processing algorithms. The added size, cost, and power consumption make it infeasible for a mobile-user device that has strict limitations on size, cost, and power consumption. Further, GPS-based synchronization requires direct line-of-sight signals from multiple GPS satellites, which are often blocked by buildings, trees, etc., therefore consistent performance is not guaranteed.